The easiest way to base coat weather is some rubbing alcohol I use 70% and black india ink mix heavy on the alcohol light on the ink for a nice base wash then go heavier on the ink for the detail the nice thing about this method is if you dont like it re-dab on another mixture and you will have a clean slate play till you like it once the alcohol evaporates the ink stays permanently Unless you add alcohol to it and once again clean slateThanks JerryJust experiment with different mixtures

btw it isnt weathered it does look amazingbut its supposed to be 200+ years old and in a wasteland at thatif you use acrylic paint and dab a bit then rub it around with a paper towel start with black then a dark brownish green then a very dark rust color